Tachometric telescope.



UNITED sTATEs PATENT oEEIcE.

HEINRICH WILI), 0F JENA, GERMANY, ASSIGNOR T0 THE FIRM 0F CARL ZEISS, 0F

JENA, GERMANY.

TACHOMETRIC TELESCOPE.

Specicatioof Letters Patent.

Patented Feb. 6, 1912.

To all whom t may concern.'

Be it known that I, HEINRICH WILD, a citizen of the German Carl-Ziess strasse, J enain the Grand Duchy of Saxe-Weimar, Germany, have invented'a new and useful Tachometric Telescope, of which the following is a specification.

The invention relates to tachometric telescopes (z'. e. stadia telescopes used for rapid surveying.) a telescope according to the advantageous arrangement, wellknown for other telescopes: that neither the` objective nor the system of marks arranged in the focal plane of the objective is shiftable in the direction of the telescope, but that the adaptation to objects at different distances is carried out by shifting a collect-ive or dispersive lens,

disposed between the .objective and the system -of marks-and in such a manner,y that the advantage given by Porro to the tachometric telescopeI after Reichenbach, viz. that by the said adaptation the tachometric (parallactic) angle changes neit-her its magnitude nor its position, is also attained. In order to approximate tosuch an unchangeableness of the tachometric angle suiiciently closely, the dimensions of .and fixed distances between the objective, the tachometric (parallactic) pair of marks and the lens are chosen in such tance from the lens to the pai-r of marks and that from the lens to the image, vwhich is projected by the objective, of that point, in which-strictly speaking only in consequence of this setting-the apex of the tachometric angle lies, is about the same, when the lens is set for very distant objects, and that the greater' of these two distances (from the lens to the pair of marks, when the lens is collective, vor from the lens to the image of the apex, when the lens is dispen sive) 'is not more than three times the smaller one, when the lens is set for the nearest objects to be measured (say about 5 m.. from the telescope). The said dimensions and xed distances may further be chosen in such a way, that, according tothe precedent of Porro. the apex of the tachometric angle is located in the tilting axis and so, at the same time, in the vertical axis of rotation of the tachometer. If the advantage due to this arrangement is waived, another one may be adopted for it. If to theconditions Empire, residing aty Its object is to construct such a manner, that the dis-` .tance from the `lens to v c c is not more than three times the distance pendent of thesmall alterations in the focalv length, to which the objective is subject. The distance from the lens to the pair of marks and that from the lens to the objective is in this case about the same, when the lens is set for very distant objects, as the image of the apex of t-he angle, projected by the objective, as well .as the apex itself, lies in t-he objective.

In the annexed drawing: Figure l is a diagrammatic view of a section along the optical axis of a t-achometric telescope constructedaccording to the invention. 2 is a diagrammatic view of a section along the optical axis of a 'second form of such a. telescope.

In Fig. 1 the apex of the tachometric angle u lies in the tilting axis indicated by thejournal a. The shiftable lens is a co1- lective lensb. Its position, in which its I distance from the tachometric pair of marks c c is'about the same as its distance from the image ofthe apex f of the angle u,

Vprojected by t-he objective d, corresponds to the setting for very distant objects, its position indicated by dotted lines to the setting for the nearest objects. The principal rays passing through e of the pencils producing the image points in c c would, las the dotted lines indicate, pierce the plane of the marks c c in gig, if the lens o were shifted as far as e. To maintain during such shifting the tachometric angle u as regards position and magnitude would require a gradual increase in the distance apart of the marks from the dimension o o to the dimension g-g. But this increase is, as long as the lens b `be not shifted beyond t-he position shown in dotted lines, very small, so that the minimum distance' c--c between the `marks need not be increased at all when adapting the telescope to the nearest objects, provided that the focal length of the lens is so small, that, when for this adaptation the lens has been shifted into the position shown ,in dotted lines,the disthe plane of the marks from the lensto e. Then to the fixed disof the dimensions and fixedA "tance cf-o between the marks there corresponds a tachometric angle u of almost un- Y optical center of the objective h.

changeable magnitude and position.

In the tachometric'telescope according t0 Fig. 2 the 'points `e and f both lie in the The shiftable lens is a dispersive lens z'. After the 'lens has been set for very distant objects,

the distance from the lens to the objective is about the same as that from the lens to the 'plane of the marks@ c. As the principal rays Aof the pencils producing image points in 0 c pass through the optical center of theobjeetive, the directionof the said principal rays ldoes not change for a small variation ofthe focal length of this objective. It willbe understood, that the small shifting ofv the lens i, which isV requisite for compensating the alteration i'n the focal length ofthe objective, does not forf itself.

of an accidental valteration in the focal length of the *objective h. Y l

I claim: 1. In a tachometric telescope, an objective, a tachometric pair of marks and a lens shiftably` fitted between the objective and the said pair of marks, the distance from the lens to the pair of marks and that from the lens to the image, which is projected by the objective, of that point in which' the apex of the tachometric angle lies, being about the same, when the .lens is set for very distant objects, and the greater of these two distances vbeing not more than three times the smaller one, when the lens is set for the nearest objects.

' 2. In a tachometrc telescope, an objective, a tachometric pair of marks and a lens shiftably fitted between the objective and j the said pair of marks, the apex of the tachometric angle lylngvin the objective, the

distance from the lens to the pair of marks' and that from the lens to the objective being about the same, when the lens is set for. very distant objects, and the greater of these two distances being not more than threeV times the smaller one, when the lens is set for the nearest objects.

HEINRICH WILD. Witnesses:

PAUL KRUGER, ALFRD MACKEDANZ. 

